1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor having dual horizontal transfer registers.
2. State of the Prior Art
Solid-state image sensors of the interline type normally include an array comprised of columns and rows of photodetectors. A so-called vertical shift register is located adjacent each of the columns of photodetectors, and charge carriers generated in the photodetectors are transferred to the vertical shift registers during each frame time. The detected image is then shifted down in unison and transferred to a horizontal shift register one line at a time. The horizontal shift register delivers the charge carriers in each line to signal processing circuitry before the next line is shifted in.
One of the problems in arrays of the type described above is to provide for a fast and efficient transfer of charge carriers from the photodetectors to the signal processing circuitry. One partial solution to this problem is to provide dual horizontal transfer registers, as shown, for example, in U.S. Pat. No. 4,750,042. The image sensor in this patent comprises a plurality of light-receiving elements arranged in a matrix, vertical CCD's for transferring charge carriers generated in the light-receiving elements out of the matrix, and two horizontal transfer registers connected to the image receiving portion through a transfer gate. The two horizontal transfer registers are connected in parallel with each other through a gate electrode so that charge carriers can be transferred from one register to the other. When both registers are full, the charge carriers are transferred to output circuitry by means of common clocking signals. A disadvantage of the image sensor shown in this patent is that separate gate electrodes are required between the two horizontal transfer registers. This makes the device more difficult to manufacture, and makes control of the device more difficult, since separate clocking signals must be provided to each of the gate electrodes.